1. Field of the Invention
Embodiments of the present invention relate to an exhaust gas control system for an internal combustion engine.
2. Description of Related Art
There is known a technique for arranging an NOx storage reduction catalyst (hereinafter, also referred to as NSR catalyst) in an exhaust passage of an internal combustion engine. The NSR catalyst stores NOx contained in exhaust gas when the concentration of oxygen in exhaust gas flowing into the NSR catalyst is high, and reduces the stored NOx when the concentration of oxygen in exhaust gas flowing into the NSR catalyst is low and there is a reducing agent.
Sulfur poisoning of the NSR catalyst occurs because of a sulfur component that is contained in fuel. There is known a technique for, when there is a request to stop an internal combustion engine, actually stopping the internal combustion engine after sulfur poisoning of the NSR catalyst is eliminated (see, for example, Japanese Patent Application Publication No. 10-231720 (JP 10-231720 A)). With this technique, the internal combustion engine is caused to operate at a rich air-fuel ratio for the purpose of eliminating sulfur poisoning.